Over the past decade, much research has been devoted to developing new and useful devices for delivering beneficial agents to agent receptor environments of use. For example, in U.S. Pat. No. 3,760,984 issued to Theeuwes, there is disclosed an osmotic delivery device comprising an inner collapsible container carrying on its outer surface a layer of an osmotic solute and a surrounding layer of a polymer permeable to fluid and impermeable to solute. In U.S. Pat. No. 3,971,376 issued to Wichterle, a device is disclosed comprising a capsule having a unitary wall formed of a substantially noncollapsible elastic material that maintains a constant volume and which is adapted to be implanted subcutaneously.
In U.S. Pat. No. 3,987,790 issued to Eckenhoff et al., there is disclosed another osmotic delivery device which contains an outer shape-retaining membrane which surrounds a solute layer which in turn surrounds a collapsible bag containing a liquid drug. Water from the exterior environment permeates through the outer membrane into the solute layer, causing the solute layer to swell The swelling forces the bag to collapse and deliver the drug through a delivery orifice. U.S. Pat. No. 3,995,631 issued to Higuchi et al., discloses a similar device (FIG. 4) comprising an inner flexible bag containing a drug formulation. The bag separates the drug from an osmotically effective solute material. Both the drug and the solute are contained within a housing having an exterior wall that is, at least in part, semipermeable. U.S. Pat. No. 3,995,632 issued to Nakano et al discloses a similar device which incorporates a movable barrier within the housing. The barrier divides the housing into two compartments, one containing the solute and the other containing the drug. The solute-containing compartment has an exterior wall that is, at least in part, semipermeable. The solute-containing compartment acts as an osmotic driver for the device.
U.S. Pat. No. 4,410,328 issued to Theeuwes discloses an osmotically driven pump. The pump includes a housing 11 and a slidable piston 14 which divides the interior of the housing into two compartments. The first compartment contains a liquid drug 13 while the second compartment contains an osmotic driver 17. The osmotic driver is a tablet of an osmotically effective solute, such as sodium chloride, which is coated with a semipermeable wall material. A delivery orifice 20 is drilled through the semipermeable wall. The bottom surface of the osmotic driver 17 is exposed to water from the external environment. This can be done by incorporating a screen, or a plurality of holes through retaining member 18, immediately below the osmotic driver 17. Water from the external environment then contacts and is imbibed through the semipermeable wall 23 into the osmotic driver 17 to form a solution of the osmotic solute. As water continues to be imbibed through the semipermeable wall, the solution in the driver is pumped through delivery orifice 20. The pumped solution eventually fills the space between the osmotic driver 17 and the bottom of piston 14. Thereafter, further pumping of solution from driver 17 forces piston 14 to more forward in housing 11. As the piston moves towards the opening 15, it urges a beneficial agent, such as a drug, out of the compartment 13.
The device illustrated in U.S. Pat. No. 4,410,328 has several potential disadvantages when used as an implantable pump. First, there is a substantial delay between the time when the osmotic driver 17 is exposed to the external aqueous environment (i.e., implanted in the body) and the time when the pump begins dispensing drug. This delay is in part caused by the space surrounding the driver which must first be filled with solution pumped from the driver before the piston is forced to move forward. Secondly, only the bottom surface of osmotic driver 17 is exposed to the external aqueous environment. Because only a relatively small portion (i.e., about 25% or less) of the membrane surface area is exposed to the external aqueous environment, the rate at which water is imbibed through the semipermeable wall 23, and therefore the rate at which the drug is delivered by the pump, is difficult to control. This is especially critical with osmotically driven pumps which are implanted in an animal body. When implanted in certain animal tissues, portions of the external surface of the pump will not be exposed to water. Thus, a dispensing device which utilizes an osmotic engine having only a small surface area of microporous and/or semipermeable membrane exposed will tend to have a greater degree of variability in its drug delivery rate since part or all of the membrane may not be exposed to aqueous body fluids.
While the above-described devices are useful for delivering many agents, and while they represent a valuable contribution to the delivery art, there has been a need in the art for an osmotically driven syringe which begins delivering a drug or other beneficial agent as soon as the syringe is placed in an aqueous environment (i.e., with a minimal lag time between the time when the syringe is placed in the aqueous environment and the time when the syringe begins delivering the beneficial agent).
It is another object of the present invention to provide an osmotically driven syringe which can delivery a beneficial agent, such as a drug, at a more uniform and more predictable delivery rate than was achieved by the osmotically driven syringes of the prior art.
In particular, it is an object of the present invention to provide an osmotically driven syringe having the above-described features and having a size making it suitable for implantation in an animal body and in particular for implantation in a human body.